1. Field of the Invention
The present invention relates to the variation of the supply power of a load supplied from an approximate D.C. voltage obtained by rectifying an A.C. voltage and, more generally, to the transmission of analog information over an A.C. supply line of a load with a capacitive input impedance.
2. Discussion of the Related Art
When it is desired to vary the supply power of a load having a resistive input impedance, a phase angle or switched phase angle variator is generally used to modulate the power transmitted to the load.
FIG. 1 shows an example of a conventional switched phase angle variator 1. Variator 1 is based on the use of a switch 2 (here a MOS transistor) connected in series with a measurement resistor Rm between two rectified output terminals 3 and 4 of a diode bridge D1, D2, D3, D4. A first A.C. terminal 5 of the bridge is connected to a terminal 6 (for example the line) of an A.C. power supply, for example mains voltage Vac, while a second A.C. terminal 7 of the bridge forms an output terminal providing a modified A.C. supply voltage Vin for the load. The other line (neutral 8) of the A.C. power supply is not interrupted. Transistor 2 is controlled by a converter 9 (for example, circuit TS555 sold by SGS-Thomson Microelectronics) connected between terminal 4 and, via a supply resistor Ra, terminal 3. Converter 9 converts a resistance variation (potentiometer 10) into a conduction time of switch 2.
The operation of a variator such as shown in FIG. 1 is well known. Circuit 9 receives a set-point order, for example, from a potentiometer 10 which sets the phase angle of the input voltage Vin. At each halfwave of voltage Vac, voltage Vin starts following the evolution of voltage Vac until switch 2 turns off under control of converter 9.
Although a variator based on such a phase switching is well adapted to applications in which the load to be supplied is of resistive type and does not require a supply from a D.C. voltage recovery, such a phase angle variator raises several problems in the case of a capacitive input impedance load.
A first problem is that, for the phase angle variation to translate as a power variation of the load, the approximate D.C. load supply voltage has to follow the power variations related to the phase angle variation. Now, the load is generally supplied via an electronic circuit drawing its own supply from the approximate D.C. voltage obtained from voltage Vin. For example, in an application to the supply of a fluorescent lamp by means of an electronic circuit forming a switched-mode converter, a variation of the approximate D.C voltage obtained from voltage Vin adversely affects the proper operation of the switched-mode converter.
Further, a switching in the charge area of a capacitor constituting the input impedance results in a significant Rms current, which is not desirable.
Independently from the input impedance of the load, a variator such as illustrated in FIG. 1 has another disadvantage which is to generate a significant dissipation due to the switching of switch 2 (for example, a MOS transistor) when said switch conducts the current for the load.
Accordingly, especially for loads having a capacitive input impedance, other means than the phase angle variation are conventionally used to act upon the load operation.
FIG. 2 very schematically shows the electric connection of a system 11 for supplying a load 12 (Q) from an A.C. voltage Vin, having a bridge 13 rectifying voltage Vin and a circuit 14 for supplying of load 12 from an approximate D.C. voltage Vout. Voltage Vout is taken across a capacitor C receiving a rectified A.C. output voltage from bridge 13.
Input voltage Vin of system 11 generally is, for a load having a capacitive input impedance such as shown, the unmodified A.C. supply voltage Vac, for example the mains voltage. The power variation function is generally performed from an analog low voltage input E of circuit 14. The signal applied to terminal E is used, for example in an application to a fluorescent lamp, to modify the frequency of the alternating current provided by switched-mode converter 14 to vary the light intensity. This light intensity dimming control terminal E is meant to be controlled by an external variator 15 setting a control voltage generally included between 0 and 5 volts and proportional to the desired light intensity.
A major disadvantage of this variation solution is the need for a low voltage link 16 between system 11 controlling load 12 and a generally remote mechanical potentiometer-switch (variator 15). As illustrated in FIG. 2, in addition to the two conductors (line and neutral) of A.C. supply Vin, two low voltage conductors (link 16) indeed have to be provided between switch 15 including a dimmer and electronic system 11 (more specifically circuit 14) for controlling load 12.
Another conventional solution to transmit a light intensity order to a load supply control circuit 14 consists of performing a carrier current modulation, that is, modulating the alternating supply current with a high frequency signal transmitting the order (for example, of light intensity). Such a solution requires, on the side of dimmer 15, a carrier current modulation system (not shown) to transmit the order and, on the side of electronic system 11, a demodulator (not shown) for extracting the power order from the A.C. supply.
Such a solution has the advantage of avoiding the use of an additional low voltage link 16. However, it has the disadvantage of being of particularly complex and expensive to implement.
UK Patent Application 2,298,553 discloses a remote control system wherein the A.C. supply is interrupted for a predetermined period immediately following a current reversal to transmit an order (0 or 1). The switch does not transmit an analog order.
The present invention aims at providing a novel solution to transmit an order to a load supply circuit which overcomes the disadvantages of conventional solutions.
The present invention aims, in particular, at providing a simple solution which requires no additional link between the control element and the load supply circuit.
The present invention aims, in particular, at providing a transmitter of an operating order over an A.C. supply line which does not create any dissipation in the transmitter.
More generally, the present invention aims at providing a transmission of an analog order by using the A.C. voltage as a transmission support, without generating any heat dissipation in the transmission and without requiring the use of a high frequency modulation transmission-reception system.
To achieve these and other objects, the present invention provides a transmitter of an analog order over an A.C. supply line for a load, including a one-way conduction element in parallel with a resistive element of variable value that is a function of the analog order to be transmitted.
According to an embodiment of the present invention, the resistive element and its value variation range are chosen to reduce or minimize the power dissipation when conducting a current.
According to an embodiment of the present invention, the one-way conduction element is formed of a diode.
According to an embodiment of the present invention, the resistive element of variable value is formed of a potentiometer.
According to an embodiment of the present invention, the resistive element of variable value is formed of a MOS transistor, the diode then being, preferably, formed by the intrinsic diode of the MOS transistor.
The present invention further provides a method for transmitting information over an A.C. supply line of a supply circuit for a load having a capacitive input impedance including a capacitor adapted to provide an approximate D.C. voltage Vout, and which includes:
on the transmitter side, of varying the peak amplitude of the A.C. voltage every other half-wave; and
on the receiver side, of sizing the capacitor so that it can withstand a single-halfwave supply and of extracting from the A.C. voltage an information proportional to the amplitude variation.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.